An analog front end (AFE) is widely used. The AFE includes correlated double sampling (CDS) that removes noise from a signal outputted from a charge coupled device (CCD) imaging element; dark current correction; automatic gain control (hereinafter, AGC); and an analog digital converter (ADC) that converts the signal to a digital video signal Vi. The ADC grayscale of the AFE is conventionally 10 bits, but 12 bits and 14 bits have become common. Furthermore, there has been advanced improvement in a complementary metal oxide semiconductor (CMOS) imaging element that allows high-speed reading by integrating a drive circuit and a read circuit.
Furthermore, with the advancement of integration of a digital signal processing circuit, not only a memory-integrated digital signal processor (DSP) dedicated for video, but also an inexpensive, generic field programmable gate array (FPGA) can easily implement the storing of output signals from a plurality of lines and the performing of arithmetic processing. Megapixel cameras with over one million pixels, high definition television (HDTV) cameras, high-speed imaging HDTV cameras, HDTV cameras with a recording unit, HDTV cameras with an Internet Protocol (hereinafter, IP) transmitting unit, ultra-high definition televisions (UHDTVs) for higher definition 2K×4K cameras or 4K×8K cameras, and uncompressed recording devices using a hard disk drive (HDD) have also been put into commercial production. For two-dimensional video display devices, too, there has been advancement in higher definition 2K×4K or 4K×8K UHDTV display, high-speed display, and ultra-slimming down.
Since the refractive index of the lens varies depending on the wavelength of light, the focal length also varies depending on the wavelength of light. Since the focal length of the lens varies depending on the wavelength, there occur axial chromatic aberration where the position of an image plane is shifted back and forth depending on the color, and magnification chromatic aberration where the magnification of an image varies depending on the color and thus the size of an image varies depending on the color.
In addition, due to spherical aberration where the position in an optical axis direction of a focal point varies depending on the distance of an incident point from an optical axis, the modulation factor of the entire screen is reduced. Due to coma (comet-like) aberration where light emerging from one point outside the optical axis does not converge to a single point on the image plane, a formed image is spread out on one side in a radial direction like a coma (comet). Thus, on the periphery of a screen, the way the contour is distorted differs between the outward and inward radial directions. Furthermore, due to astigmatism where an image point in a concentric direction and an image point in a radial direction by a ray of light emerging from one point outside the optical axis are shifted from each other, on the periphery of the screen, the way the contour is distorted in the circumferential direction differs from the way the contour is distorted in the radial direction.
The spherical aberration is proportional to the third power of the numerical aperture (NA) and independent of the size of the field of view, and is the one and only aberration that appears even at the center of the screen. The spherical aberration of a lens doublet composed of two lenses in which the refractive index of a concave lens is higher than that of a convex lens is reduced by one digit or more over a single lens. In addition, the coma aberration is proportional to the second power of the open area ratio NA which is the reciprocal of the aperture ratio F, and to the first power of the size of the field of view, and on the periphery of the screen, the way the contour is distorted differs between the outward and inward radial directions. In addition, the astigmatism is proportional to the first power of NA and to the second power of the size of the field of view.
A phenomenon where light collected by a lens does not focus on a single point is called aberration, and a lens that is corrected for spherical aberration and coma aberration among aberrations is called an aplanat, and furthermore, a lens in which a focal position shift caused by different wavelengths of light is corrected at two locations, i.e., the red C-line (656.3 nm) and the blue F-line (486.1 nm), is called an achromat which is an achromatic lens. A lens that satisfies conditions that, for example, chromatic aberration is corrected at three wavelengths where the violet g-line (435.8 nm) is further added, and spherical aberration and coma aberration are corrected at two wavelengths is named as an apochromat by Abbe.
A high-power zoom lens (e.g., an 88× box or 42× cylindrical zoom lens) that is often used in relay broadcasting is easy to correct for spherical aberration and coma aberration at two wavelengths for an intermediate focal length, but is difficult to correct for spherical aberration and coma aberration at two wavelengths for the wide-angle and telephoto ends. A lens corrected for spherical aberration and coma aberration at three wavelengths is large in size and expensive, like a lens for movies, even for a unifocal lens or a low-power zoom lens. A high-power zoom lens that is corrected for spherical aberration and coma aberration at three wavelengths is very large in size and very expensive and thus has not been put into commercial production.
A lens that is not even an aplanat due to its insufficient correction of spherical aberration and that has a reduced modulation factor even at the center of the screen is insufficient in performance for UHDTVs.
Meanwhile, remaining aberration varies by different aberration correction methods.
In addition, a catoptric system uses reflection of light by a mirror, instead of using refraction of light by glass like a lens. As a result, it is easier for the catoptric system to achieve a large aperture and a high resolution than it is for an optical system including only a lens. Hence, the catoptric system has started to be heavily used not only in large-aperture reflecting telescopes having an aperture on the order of 0.2 m to 10 m and ultra-telephoto reflex lenses for single-lens reflex cameras that have a focal length on the order of 500 mm to 2000 mm, but also in optical systems for semiconductor pattern printing that use an ArF laser with a wavelength of 193 nm or use extreme ultraviolet light with a wavelength of 10 nm or less. However, the catoptric system such as a reflecting telescope does not have chromatic aberration but has coma aberration regardless of whether it is of a Newtonian or Cassegrain type. Thus, if a lens that corrects coma aberration is added to the catoptric system, it becomes large in size and expensive.
Meanwhile, in an imaging device including a lens, an imaging element, and a video signal processing circuit having a contour correction function, there are eight or more line memories, a vertical contour correction signal is generated from each of a plurality of video signals which are delayed by an integer horizontal period, there are eight or more pixel delay functions, a horizontal contour correction signal is generated from each of a plurality of video signals which are delayed by an integer pixel quantity, and upon checking, the vertical contour correction signals and the horizontal contour correction signals are added to the video signals (see Patent Literature 1).
In addition, there is also an imaging device that performs an image sharpening process such as an aperture correction process or an edge enhancement process only for a concentric direction of an image having been subjected to distortion aberration correction by image processing, and does not perform the image sharpening process for a radial direction of the image (see Patent Literature 2).